FOC images are routinely calibrated using three sets of calibration
files: flat field images, geometric correction files, and instrument sensitivity
tables. Full descriptions and histories of these files and tables can be found in the
FOC Instrument Science Report
FOC-082 [Postscript, 150 kb]).
All reference files can be obtained from the HST Archive.

DQE File

The FOC DQE curve has recently been updated
based on Cycle 4 calibration observations. The new DQE curve corresponds to the curve found
in Versions 5-7 of the FOC Instrument Handbook and is coupled with the encircled energy curves
found there. An FOC Instrument Science Report FOC-085
[Postscript, 386 kb] describes this new DQE curve and it calibration.

Flat Field Images

Unsmoothed versions of the flat fields
for the f/96 relay and the f/48 relay illustrate the locations of small
scale features in FOC images which were not removed in the 'pipeline'. An
unsmoothed version of the f/96 flat field using the latest geometric correction
has been made available for comparison with
post-COSTAR f/96 images.

Reseau Positions

FOC Reseau positions for the pre-COSTAR
f/96 and f/48 relays are available. These files can be used for comparison with
FOC images to determine whether a source has fallen onto a reseau. They also
provide an indication of the amount of geometric distortion that is corrected
in FOC images.

Objective Prism Calibrations

Objective Prism Reduction Techniques and Software:

An Instrument Science Report (FOC-092) [Postscript, 738 kb]
has been written to describe the most recent calibrations of the objective
prisms. This report also describes the techniques and available software for
extracting a spectrum from an image and reducing the spectrum. This tutorial will be
useful for anyone interested in using the objective prisms or for anyone with objective
prism images already.

Wavelength Calibration:

Dispersion curves provide the relation between wavelength and offset in
pixels from the undispersed image. These curves are available
as ASCII tables for the following prisms:

The dispersion curve for the f/96 NUVOP has been re-calibrated based on
post-COSTAR observations of a spectro-photometric standard and a wavelength
standard. As a result, this curve will wavelength calibrate a NUVOP image
to within 0.5% of the wavelength below 3800 Angstroms, and 1% from 3800 to 6000
Angstroms, as long as the position of the undispersed target is known to within
a few pixels (less than 4).

Photometric Calibration:

Software has been written for the STSDAS package to
wavelength calibrate and photometrically reduce objective prism data.
The reduction software has been simplified into one routine:
objcalib. After extracting a spectrum from an unrotated image
using the apextract package, objcalib will produce a
flux- and wavelength-calibrated 1-D spectrum. For f/96 NUVOP spectra,
the photometry can have errors as low as 10% below 3800 Angstroms with
the new dispersion curve, and possibly as low as 20% from 3800 to 6000
Angstroms.

Many PSFs have been taken with the FOC to characterize the shape of
point sources in all wavelengths visible to the FOC. A
library of observed PSFs
for all relays has been established to assist users in
analyzing or reducing their FOC data.

Calibrations comparing f/96 images of a standard star taken with
different filter combinations have measured the
image shifts
induced by the individual filters. For most UV filters, this image
shift is negligible, being less than 1 pixel. However several filters,
primarily in the visible, induce shifts of 0.1" (7 pixels) or more.
Fortunately, none of the ND filters showed that they induced any
significant shifts. These shifts should be taken into account if
planning an INT ACQ, as it would introduce extra error in the final
position of the target, especially if a small aperture is being used.